The invention relates to an antenna on board a spacecraft, such as a geosynchronous satellite, adapted to receive and/or transmit radio frequency signals such as radio communication signals or radar signals.
A geosynchronous satellite comprising a transmit antenna and a receive antenna, each of which has a reflector associated with a multiplicity of radiating elements, also known as sources, is used to provide communications over an extended territory, for example a territory the size of North America. To enable re-use of communication resources, especially frequency sub-bands, the territory to be covered is divided into areas and resources are allocated in such a way that adjacent areas are allocated different resources.
Each area, which has a diameter of the order of several hundred kilometers, for example, is of such a size that it must be covered by a plurality of radiating elements, in order to provide a high gain and so that the radiation from the antenna is sufficiently homogeneous over the area.
FIG. 1 shows a territory 10xe2x80x2 covered by an antenna on board a geosynchronous satellite and n areas 12xe2x80x21, 12xe2x80x22, . . . , 12xe2x80x2n. In this example, four frequency sub-bands f1, f2, f3, f4 are used.
The area 12xe2x80x2i is divided into a plurality of sub-areas 14xe2x80x21, 14xe2x80x22, etc., each of which corresponds to one radiating element of the antenna. FIG. 1 shows that some radiating elements, for example the element 14xe2x80x23 at the center of the area 12xe2x80x2i, correspond to only one frequency sub-band f4, whereas others, for example those at the periphery of the area 12xe2x80x2i, are associated with a plurality of sub-bands (the sub-bands allocated to the adjacent areas).
FIG. 2 shows a prior art receive antenna for the above kind of telecommunication system.
The antenna has a reflector 20 and a plurality of radiating elements 221, . . . , 22N in the vicinity of the focal plane of the reflector. The signal received by each radiating element, for example the signal coming from the element 22N, passes first through a filter 24N for eliminating the transmit frequency (which is at a high power) followed by a low-noise amplifier 26N. At the output of the low-noise amplifier 26N, a divider 30N divides the signal into a plurality of portions, possibly with coefficients that can differ from one portion to another; the object of this division is to enable a radiating element to contribute to the formation of a plurality of beams. Thus an output 321 of the divider 30N is allocated to an area 34p and another output 32i of the divider 30N is allocated to another area 34q.
The dividers 301, . . . , 30N and the adders 34p, . . . , 34q for constituting the areas are part of a system 40 as a beam forming network (BFN).
In the beam forming network 40 shown in FIG. 2, each output of each divider 30i is provided with a combination of a phase-shifter 42 and an attenuator 44. The phase-shifters 42 and the attenuators 44 modify the radiation diagram, either to correct it if the satellite has suffered an unwanted displacement or to modify the distribution of the terrestrial areas.
Each low-noise amplifier 26N is associated with another low-noise amplifier 26xe2x80x2N which is identical to it and whose function is to replace the amplifier 26N should it fail. To this end, two switches 46N and 48N are provided to enable such replacement. It is therefore necessary to provide telemetry means (not shown) for detecting such failure and telecontrol means (also not shown) to effect such replacement.
For existing satellite xe2x80x9cmobilexe2x80x9d services (for example satellite mobile telephony services) to grow without competition from terrestrial networks, it is necessary for the terminals used for these services to have the same overall size as those used for terrestrial networks. The only parameter of the link balance that is still open to modification in order to reduce terminal size and power is the figure of merit of the satellite, in the uplink direction, and the equivalent isotropically radiated power (EIRP) transmitted by the antenna of the satellite, in the downlink direction. To increase the EIRP of the satellite, it is possible to find a compromise between the size of the antenna and the power of the satellite amplifiers. However, a compromise is not possible for the figure of merit, because the noise temperature is fixed by natural constraints. Improving the figure of merit must therefore be achieved by increasing the size of the antenna.
A large antenna, i.e. an antenna having a large surface area for picking up or radiating electromagnetic signals, has the benefit of a high gain (its gain is proportional to its surface area) and a corresponding resolution (its resolution is proportional to its largest dimension). The great majority of space applications, such as radio communications, eavesdropping, and electromagnetic remote sensing, require the use on board spacecraft of antennas with a very high gain and a very high resolution. This is why, at present, space applications use antennas with a very large reflector (having a diameter of the order of 12 to 15 meters).
However, producing antennas with a diameter greater than 15 meters gives rise to numerous technical and practical problems, in particular stowage in the nose-cone of the launch vehicle, deployment from the spacecraft in orbit, and various mechanical and electrical constraints associated with objects in zero gravity and a vacuum, such as structural stiffness, mechanical strength, mechanical vibration, expansion and contraction.
One solution to these problems is to use xe2x80x9cactivexe2x80x9d antennas with arrays of deployable radiating elements.
One such antenna, described in U.S. Pat. No. 5,430,451, is an array antenna for spacecraft including a plurality of sub-arrays connected together by a mechanism with joints. In this way, the antenna can occupy a folded configuration (referred to as the stacked configuration) during launch of the spacecraft and a flat, unfolded configuration (referred to as the unstacked configuration) after the spacecraft is launched.
However, establishing coherence of the signals from the sub-arrays does not take account of mechanical deformation relative to each other of the panels supporting the sub-arrays.
An object of the present invention is to eliminate the drawbacks previously cited. The invention has the particular object of providing a simple way to obtain a wide active array antenna comprising a plurality of deployable sub-arrays of radiating elements.
To this end, the invention provides a beam forming network adapted to cooperate with an active array antenna of a spacecraft, the antenna including:
a plurality of sub-arrays of radiating elements, and
a plurality of support panels for supporting respective sub-arrays, which panels are able to move from a folded configuration in which the panels at least partially overlap to a deployed configuration in which the panels are substantially coplanar,
said beam forming network including means for establishing the coherence of respective signals received by the plurality of sub-arrays by weighted summation of said signals as a function of the expected angle of incidence (xcex8) on the sub-arrays of the respective signals and the expected relative phase-shifts due to signal propagation time-delays between the sub-arrays, and said beam forming network further comprising means for estimating information representative of a deformation (xcex1) of the relative positions of the panels compared to an expected predetermined configuration, and said summation of said signals is also effected as a function of said information representative of deformation.
Establishing coherence of the signals received by the sub-arrays entails weighted summation of the signals. The weighting applied to each signal is calculated as a function of the required angle of incidence of the signal on the sub-array, the real (or observed) angle of incidence of the signal on the sub-array, and the phase-shifts due to relative signal propagation time-delays caused by the relative positions of the sub-arrays and the distances between them.
Coherent summation of the payload signals uses information on the relative geometry of the panels.
Using a plurality of sub-arrays of radiating elements and associated support panels has the advantage of a stackable structure that can be accommodated within a volume compatible with that of a launch vehicle nose-cone.
Deploying the stacked structure does not necessitate any complex opening-closing mechanism. For example, opening and closing can be effected in the conventional manner used for solar panels. The support panels do not require any mechanical stiffness in their connection to the spacecraft. Furthermore, the absence of a locking system and the freedom of movement (possibility of oscillation) between adjacent panels reduces the mechanical stresses on the spacecraft.
In one embodiment of the invention, the beam forming network according to the invention includes digital signal processing means.
In one embodiment of the invention, the digital signal processing means include computation software.
In one embodiment of the invention, because said radiating elements are adapted to be employed for receiving and transmitting signals alternately or simultaneously, each radiating element of the panels is connected to respective phase-shifter means adapted to modify the phase of the wave to be transmitted, and the beam forming network includes respective control means for controlling said phase-shifter means so that said deformation is compensated by the modification of the phase of the respective radiating elements of the panels in deformed positions.
The invention also provides a system for receiving radio frequency signals comprising a radio frequency antenna for spacecraft and a beam forming network, the antenna comprising:
a plurality of sub-arrays of radiating elements, and
a plurality of support panels for supporting respective sub-arrays, which panels are able to move from a folded configuration in which the panels overlap at least partly to a deployed configuration in which the panels are substantially coplanar,
and the beam forming network including means for establishing the coherence of respective signals received by the plurality of sub-arrays by weighted summation of said signals as a function of the required angle of incidence of the respective signals on the sub-arrays, the actual angle of incidence of the signal on each sub-array, and the relative phase-shifts due to signal propagation delays,
wherein said beam forming network is a network according to the invention.
In one embodiment of the invention said plurality of panels comprises first and second series of panels for receiving and transmitting radio frequency signals, said system includes a multiple-source transmitter system adapted to transmit the transmit signals toward the second series of panels, which include radiating elements corresponding to each source, each corresponding radiating element being adapted to receive a specific signal intended to be phase-shifted by said phase-shifter means as a function of the deformation information received by the network, and the signal, phase-shifted in the above manner where applicable, is transmitted to the respective radiating element of the first series of panels for radio transmission.
In one embodiment of the invention, the analog means for processing the receive and transmit radio frequency signals are on the panels.
In one embodiment of the invention, said analog processing means are connected to the beam forming network by at least one optical fiber.
The invention further provides a spacecraft including a system in accordance with the invention for receiving radio frequency signals.
The invention further provides a beam forming method for use in a beam forming network adapted to cooperate with a radio frequency antenna on board a spacecraft, said antenna comprising:
a plurality of sub-arrays of radiating elements,
a plurality of support panels for supporting respective sub-arrays, the panels being able to move from a folded configuration of the antenna in which the panels at least partly overlap to a deployed configuration in which the panels are substantially coplanar,
said method including a step of establishing the coherence of respective signals received by the plurality of sub-arrays by weighted summation of said signals as a function of the expected angle of incidence on the sub-arrays of the respective signals and expected relative phase-shifts due to signal propagation delays, which method further includes, before the step of establishing coherence, a step of estimating information representative of a deformation of the relative positions of the panels relative to an expected predetermined configuration,
and said summation of said signals is also effected as a function of said information representative of deformation.
In one embodiment, said information representative of deformation comprises the angle between said two adjacent panels, said angle being used for the summation.
In one embodiment of the invention, said method includes a step of a remote beacon signal transmitter whose location is known transmitting a beacon signal to enable estimation of said information representative of a deformation relative to said expected predetermined configuration.
The invention further provides a system comprising:
a spacecraft according to the invention, and
at least one remote beacon signal transmitter whose location is known to said spacecraft to enable estimation of said information representative of a deformation relative to said expected predetermined configuration.